1. Field of the Invention
The present invention relates to a printing apparatus and method for performing printing with an optimum spacing maintained between the surface of a printing medium and a print head.
2. Description of the Related Art
Printing apparatuses print information such as characters and images on a printing medium such as a sheet of paper, fabric or a plastic sheet. An ink jet printing method, as a nonimpact type printing method, projects ink droplets onto the surface of a printing medium through ink nozzles, permitting high-density and high-speed printing. For these advantages, the ink jet printing method is widely used in printing apparatus of a diversity of pieces of office equipment including printers, photocopying machines, facsimile machines, and wordprocessors.
The-printing apparatus that uses such a ink-jet printing method needs to keep constant the spacing between the printing surface of a printing medium and an ink jet print hand to form an optimum image on the printing surface. With an ink-jet head held to the printing surface of a printing medium with too narrow spacing therebetween, the ink-jet head may contact the printing surface of the medium, smearing the printing surface, or the head itself is possibly damaged. When the spacing between the ink-jet print head and the printing surface of the medium is too wide, the image quality may be degraded.
The printing media used in the ink-jet printing apparatuses include not only particular types of paper, but a diversity of media including envelopes, postcards, overhead projector sheets, and fabrics. As the types of printing media vary, their thicknesses vary accordingly, and the spacings between the printing surfaces of the printing media and the ink-jet print head also vary. As a result, the printing surface of the medium is smeared, the ink-jet head is damaged, and the resulting image quality suffers degradation.
In an attempt to preclude these problems, the ink-jet print head is shifted in accordance with the thickness of the printing medium using a lever to maintain an appropriate spacing between the printing surface of the printing medium and the ink-jet print head. The handling of the lever is a clumsy and delicate operation, and is not a satisfactory solution.
One method of keeping constant the spacing between the printing surface of a medium and an ink-jet head regardless of the thickness of the medium has been proposed in Japanese Unexamined Patent Publication No. 7-81047. According to this disclosure, a pair of driven rollers for advancing a printing medium is disposed on the side of an ink-jet print head that faces a platen with the printing medium being interposed between the platen and the ink-jet print head. The platen is urged toward the rollers so that the position of the printing surface of the printing medium remains fixed relative to the ink-jet print head regardless of the thickness of the medium.
FIG. 14 shows the construction of the major portion of such a known printing apparatus. As shown, an ink-jet print head 103 faces a platen 102 with a printing medium 101 interposed therebetween. The ink-jet print head 103 is mounted on a carriage 105 that slidably reciprocates along a guide rail 104, and thus prints a desired image onto the printing surface of the printing medium 101. A pair of transport rollers 107, 108 are arranged upstream of and downstream of the ink-jet print head 103 along the advance of the printing medium 101 that is moved from left to right in FIG. 14. The transport rollers 107, 108 are pressed into contact with the printing surface 106 of the printing medium 101 in a manner that the transport rollers 107, 108 are rotatable in their driving direction. A pair of pinch rollers 109, 110 that are rotatable supported are pressed into contact with the printing medium 101 against the transport rollers 107, 108. The pinch rollers 109, 110 along with the platen 102 are pressed toward the ink-jet print head 103 with the urging of pressure springs 111. As the transport rollers 107, 108 rotate to advance the printing medium 101, the pinch rollers 109, 110 run freely along therewith. The pinch rollers 109, 110 along with the platen 102 are displaced in the direction toward the transport rollers 107, 108 to keep constant the spacing between the ink-jet print head 103 and the printing surface 106 of the printing medium 101.
U.S. Pat. No. 4,620,807 discloses a wire printer 5 for printing on envelopes 12 of a wide range of thicknesses. In the disclosure, a platen 10 is displaced vertically downwardly by a link mechanism 36, 37 and 40 and a slide mechanism 51, 58 in accordance with the thickness of an envelope that is transported in a horizontal direction.
In the known ink-jet printing apparatus shown in FIG. 14, there are times when the printing medium 101 is caught between the upstream transport roller 107 and the pinch roller 109 with the forward edge of the printing medium 101 yet to reach the nip between the downstream transport roller 108 and the pinch roller 110. There are also times when the printing medium 101 is caught between the downstream transport roller 108 and the pinch roller 110 with the back edge of the printing medium 101 already parted from the nip between the upstream transport roller 107 and the pinch roller 109. In such conditions, depending on its thickness, the printing medium 101 is slightly tilted with respect to the ink-jet print head 103, and thus the spacing between the ink-jet print head 103 and the printing surface 106 cannot be kept constant.
The downstream transport roller 108 has on its circumference a spur-gear-like thin sheet with serrations to minimize contact with freshly printed ink on the printing medium 106. The pinch roller 110 displaces against the urging of the pressure spring 111, and therefore, the downstream transport roller 108 results in no sufficient friction with the printing surface 106. In the region where the printing medium 101 is advanced only by the nip between the downstream transport roller 108 and the pinch roller 110, the advance accuracy of the printing medium 101 may be degraded.
In the recording apparatus disclosed in U.S. Pat. No. 4,620,807, when a recording medium, a thin one in particular, comes into a recording area, the downstream side of the recording medium fails to shift in a vertical direction and the platen is subject to a tilt, because of looseness in mounts of links in the link mechanism. Furthermore, the use of the link mechanism makes area inevitably bulky the structure of the recording area.